In recent years, the popularity of battery-powered electronic devices, such as laptop computers, personal digital assistants, and cellular telephones, has grown dramatically. This growth, in turn, has fueled consumer demand and expectations for longer battery life, and driven manufacturers and researchers to focus more attention on improving the energy efficiency of the microprocessors and other integrated circuits that enable these devices.
Integrated circuits, also known as “chips,” are interconnected networks of electrical components, fabricated on a common foundation, or substrate, of semiconductor material. These circuits typically comprise millions of microscopic transistors. A key aspect of energy efficiency in integrated circuits is the control of leakage current in these transistors.
Leakage current refers to electric current that a transistor conducts when turned off. Ideally, this current is zero; however, in practice, all transistors exhibit some level of leakage current. (Leakage current is analogous to water that flows from a leaky faucet.) The cumulative leakage for a circuit having millions of transistors can amount to a significant amount of wasted power—known as leakage power. For example, in some circuits, leakage power may account for as much as one third of total power usage.
Although there are a number of techniques available to reduce leakage current, there is still considerable room for improvement. For example, one prevailing technique is vector control, which entails applying a single, optimized input vector (that is, a particular set of input signals) to an entire integrated circuit to lock its transistors in a collectively reduced or optimal leakage state. However, in studying this technique, the present inventors have recognized that it becomes increasingly ineffective as circuit complexity or size increases.
Accordingly, there is a need for better ways of reducing leakage current in integrated circuits, particularly larger, complex circuits, such as microprocessors.